Place cursor at start of paragraph, not heading, when section-editing

Also checks for subheadings, i.e. we keep moving forward one sibling
as long as the next sibling is a heading of higher order than the
previous one. If the heading is of the same or lower order then we
assume there is an empty section, so we just stop.

Bug: 49563
Change-Id: I6fcc463aeeae40278f5caf04ae251c8ecfe65472
Merge "Place cursor at start of paragraph, not heading, when section-editing"